Line guides of the type above referred to serve to guide the fishing line along the rod and at its top so that the line can be let out and reeled in without becoming entangled with the rod itself or otherwise. The fishing line is generally in contact with the inner surface of the guide ring of line guides. This contact may be a pressure contact, for instance, when a heavy sinker is attached to the hook carrying end of the line or a caught fish is played. The passage speed of the line through the guide ring of a line may be slow, for instance, when the fisherman just lets out the line until the bait reaches bottom or very fast when the fisherman is casting or plays a caught fish.
As it is evident from the afore-listed conditions under which a fishing line functions, the friction between the line and the running surface of the line guides is a significant factor largely controlling the useful lifetime of the line and in particular its resistance to snapping under strain. The friction factor is particularly important as the smoothness of the line surface ring may be restricted by sand or dirt particles adhering to the line. Snapping of the line while in use is not only annoying to the fisherman, but may also be quite costly.
Various attempts have been made to reduce the friction between the line and the ring of a line guide to a minimum, both as to the shape of the guide ring and the material used for its running surface. There are known line guides of rectangular configuration to provide a relatively large and flat running surface to avoid the formation of grooves in the inner ring surface as may occur when the line is confined to narrow running surfaces but line guides so shaped have relatively sharp edges even if these edges are rounded and friction is substantially increased when the line, due to the pull direction, contacts an edge under pressure. Most line guides as now used have a circular, or at least substantially circular, configuration. Such line guides have no flat running surfaces but they also do not have edges.
Various attempts have also been made to provide material for the rings of line guides which has a low friction factor, is sufficiently hard to avoid or at least reduce the formation of scratches and grooves by abrasive particles carried by the line and is not sensitive to corrosion by salt water. For instance, several types of alumina have been used for guide rings.